First, make the flavoured butter.
Place the seaweed flakes and the wild garlic in a bowl with the butter and plenty of salt and pepper. Mix to combine.
Spoon the butter into the bone cavity. Making sure it’s as evenly distributed as you can. Now tie the leg up. If you’re familiar with the ‘butchers knot’ you could use this approach, but any knot will work, as long as it doesn’t come undone. It’s a good idea to soak the string in some water. This will stop it catching as it’s sat above the fire. Season the meat all over with salt and pepper and rub with the olive oil.
Let the mutton leg come up to room temperature while you prepare the fire.
I usually make a simple spit from some lengths of hazel or driftwood, it’s a basic contraption, easily replicated by anyone who’s up for having a go. The frame consists of two uprights and a cross-section that sits over the fire from which I can suspend the meat, for this, I use a small butchers hook and some heavy string. I can adjust the height and rotate what I’m cooking. It’s an incredibly simple approach.
Start the fire with small pieces of dry kindling, twigs and sticks. When it’s burning well add some large pieces of dry wood and some charcoal if you're using it.
Let the fire build up it wants to be circular and large enough that when the mutton is hung above it, it is getting heat from all sides.
When the flames have burned down and you have some really hot embers glowing away set the stuffed mutton over the fire. It should be no more than a couple of foot above the coals – you certainly shouldn’t be able to hold your hand below the lamb meat for any length of time. Cook the meat steadily and consistently over a good hot fire, turning periodically, raising or dropping the level depending on the heat from below. Keep feeding the fire and watching, and be on hand to make small adjustments to both meat and fire. The mutton leg will take several hours to cook. I’m being specifically unspecific and rightly so. There are too many variables involved to suggest exact cooking times.
To get a good idea of how you’re getting on, pierce the meat every so often with a skewer or small knife through to its thickest part. Leave it there for 30 seconds then take it out and touch it to your wrist for an instant. If it burns, It's going to be ready.
The lamb should be piping hot throughout before you take it off to rest.
Serve slices of the mutton with fire-baked jacket potatoes and a homemade salsa verde.